Objective:The purpose of this in-vitro study was to investigate the effect of glow discharge and dielectric barrier discharge (DBD) plasma on repaired acrylic denture base resin as regard to shear bond strength and transverse strength.Materials and methods:Eighty specimens were prepared in this study, 30 for shear bond strength and 50 for transverse strength. Shear bond strength specimens are double fused discs. One disc prepared from heat-cured acrylic resin 15 mm × 3 mm. Glow discharge and DBD plasma with consumed energy of 360 and 720 J were used to treat the specimens. The surface of specimen was wetted with methylmethacrylate. The other disc, prepared from self-cured acrylic resin 6 mm × 2 mm, was attached to that surface as a repair material. Shear bond strength was determined using a universal testing machine. Transverse strength specimens were prepared in dimensions of 65 mm × 10 mm × 2.5 mm. Specimens were fractured, treated with plasma, wetted with methylmethacrylate then repaired. Transverse strength was measured by three-point bending test. Data were analyzed using one-way analysis of variance and Tukey's multiple comparison test interval at 0.05 level of significance.Results:There was a significant difference between the mean values of shear bond strength (MPa) and between the mean values of transverse strength (MPa) of repaired acrylic denture base resin (P < 0.05%). The highest shear bond strength was recorded with glow discharge at 720 J while the highest transverse strength was obtained with DBD plasma at 360 J.Conclusion:Using glow discharge plasma at 720 J as a surface treatment could increase the shear bond strength. Also, using DBD plasma treatment at 360 J could increase the transverse strength.

Polymethylmethacrylate (PMMA) has been established as the principal material in denture base construction due to its good overall processing as well as its adequate properties [1]. However, acrylic dentures are susceptible to fracture after periods of clinical use. Dentures repaired with autopolymerizing acrylic resin often refracture at the repair site. The fracture of repaired dentures often occurs at the interface junction of the original base and repair materials, rather than within these materials [2].

Attempts have been made to improve the mechanical properties of the repaired sites by changing either the joint surface contours, the processing methods [3], reinforcing materials, such as metal wires [4] or by using surface treatment. Zamperini et al. [5] have showed that the plasma as a surface treatment has potential to modify the chemical composition, hydrophobicity, and topography of denture base acrylic resin surface.

Plasma, a quasi-neutral gas, is referred to as the fourth state of matter. It consists of a collection of electrons and ions as well as neutrals, atomic and molecular species that exhibit a collective behavior in the presence of an electromagnetic field. Plasma, mainly generated by electric field could also be generated by other means, including magnetic field, combustion and nuclear reactions [6]. Nowadays, plasma science includes a variety of science fields ranging from plasma physics to divisions of chemistry, atomic and molecular physics and materials science [7].

A glow discharge is the oldest type of plasma. It is a nonthermal plasma which produced at reduced pressure and assures the highest possible uniformly and flexibility of any plasma treatment [8]. Glow discharge plasma altered the surfaces of the acrylic resin and increased the wettability as shown both by X-ray photoelectron spectroscopy and contact-angle measurements and plasma treatment seemed to offer durable (at least up to 60 days) wettability [9]. Dielectric barrier discharge (DBD) has gained importance in the medical field due to its property of being functional at room temperature and atmospheric pressure; unlike low pressure cold plasma [10]. DBD is one of the methods used to produce nonthermal or cold atmospheric plasma. Studies of cold atmospheric plasma showed promising results in tooth bleaching, bacteria inactivation, instrument sterilization, and surface treatment [11].

Surface modification by plasma treatment is achieved using gases such as air, oxygen, nitrogen, argon, and helium. The objectives of plasma surface modification in biomedical applications are adhesion promotion, enhanced surface wettability and reduced friction. Factors that contribute to improved adhesion are removal of surface contaminants and weakly bound polymer layers, etching and substitution of chemical groups on the surface that permit covalent bonding [6].

Plasma treatment seemed to be a promising technique to improve the surface wettability of denture base materials as well as exhibiting a good antibacterial effect without degrading the material's physical properties. So, plasma treatment was suggested by Pan et al.[12] as a valuable means for the surface modification of acrylic resins and prevention of denture stomatitis [13]. Plasma can also be used for tooth whitening [14]. Plasma treatment was effective in increasing the shear bond strength of self-curing resin to heat-cured acrylic resin not only for a short term but also for a long term [15],[16].

The above mentioned advantages of plasma and the fewer studies concerning the effect of different plasma treatments on repaired acrylic denture base resin have suggested studying the effect of glow discharge and DBD plasma as surface treatment on repaired acrylic denture base resin. The null hypotheses of this study are: (i) plasma treatment is not effective in enhancing shear bond strength of repaired acrylic denture base resin. (ii) Plasma treatment has no effect on transverse strength of repaired acrylic denture base resin.

Materials and Methods

Eighty specimens were prepared in this study, 30 for shear bond strength test and 50 for transverse strength test.

Shear bond strength test

Thirty specimens were prepared for shear bond strength test in the form of double fused discs. One disc prepared from heat-cured acrylic resin (Acrostone Dental and Medical Supplies, Alexandria, Egypt) which was plasma treated then wetted with methylmethacrylate (MMA) monomer. The other disc, prepared from self-cured acrylic resin which was attached to the treated surface as a repair material.

Preparation of specimens

Thirty disc-shaped specimens were prepared from heat-cured acrylic resin using compression molding technique according to the manufacturer's instructions. The specimens were processed with dimensions of 15.0 mm diameter and 3.0 mm thickness according to Sarac et al.[17]. After processing, the specimens were trimmed with an acrylic stone, polished and stored in water at 37°C for 7 days.

Grouping of the specimens

The prepared disc-shaped specimens were divided into five groups, six specimens each: Group I, specimens were not treated with plasma (control group). Group II, specimens were surface treated for 1 min with glow discharge plasma with consumed energy of 360 J. Group III, specimens of this group were surface treated for 2 min with glow discharge plasma with consumed energy of 720 J. Group IV, specimens were surface treated for 2 min with DBD plasma with consumed energy of 360 J. Group V, specimens were surface treated for 5 min with DBD plasma with consumed energy of 720 J.

Plasma treatment

Glow discharge plasma system

The schematic diagram of experimental arrangement of glow discharge plasma system (Designed at Center of Plasma Technology, Al-Azhar University, Cairo, Egypt) is shown in [Figure 1]. The discharge cell consists of two movable parallel electrodes enclosed in a vacuum vessel. Each electrode consists of a disk of copper brass of 10 cm diameter. The vessel consists of cylindrical pyrex glass tube. The tube was pumped by double stage rotary pump (Tungsram) to a base pressure of 4 Torr. The tube was connected to open air via a needle valve (Leybold AG 283 40). The flow of the air gas to the discharge tube can be controlled using the needle valve, and hence adjusting the gas pressure inside the discharge tube. A vacuum gauge (Edwards capsule dial gauge cg 16 K) was connected to the discharge tube to measure the pressure of the gas inside it. To generate the discharge, the two electrodes are connected to DC power supply (BCB-2 N) across a variable load resistance to control the discharge current. The output voltage of the power supply can be varied from 0 to 3 kV. The discharge current was measured by digital ammeter (DT-3900). The discharge voltage was measured by digital voltmeter (DT-3900) via potential divider (ratio 10: 1). The specimens were fixed on the negative electrode (lower electrode) and exposed to the generated plasma between the two electrodes. Specimens have been exposed to plasma for different treatment times (1 and 2 min).

The experimental arrangement of DBD plasma system used in the present treatment is shown in [Figure 2]. DBD cell consists of two electrodes of stainless steel disc has a diameter of 25 cm and thickness of 2 mm. The lower electrode was fixed to a Perspex base of 30 cm diameter and 2 cm thickness and connected to earth. The upper electrode was fixed to a Perspex disc of 30 cm diameter and 1 cm thickness and connected to high voltage AC power supply of 50 Hz frequency and a variable voltage of 0–20 kV. A dielectric material of glass had a thickness of 1.2 mm was pasted to the upper electrode. The upper and lower Perspex discs collected to each other via O-ring. The gap distance (d) between dielectric glass and the lower electrode was 3 mm. The cell was working in open air at atmospheric pressure. Before any treatments the air gas was left to flow in the cell for about 5 min for sweeping any impurities in the gap space. The electric circuit of the discharge system was powered by alternating current power supply. The treated specimens were fixed at the lower (earthed) electrode. Specimens have been exposed to plasma for different treatment times (2 and 5 min).

After plasma treatment, the treated surface of the specimens and plain surface of control group were wetted with MMA monomer. Piece of internal urinary catheter with dimensions of 6 mm diameter and 2 mm height was used to add the self-cured acrylic resin to the specimen surface. The self-cured acrylic resin was mixed according to the manufacturer's instructions and inserted into the plastic ring and pushed toward each disc-shaped heat-cured PMMA specimen until autopolymerization was ended, then the plastic rings were removed with a sharp scalpel. So, double fused discs were obtained each containing heat-cured acrylic resin disc to which the repair material (self-cured acrylic resin) was attached. Specimens were stored in distilled water at 37°C for 24 h before testing.

Shear bond strength testing

The shear bond strength of the specimens was determined as shown in [Figure 3] using a universal testing machine (Model LRXplus; Lloyd Instruments Ltd, Fareham, UK) at a cross-head speed of 1.0 mm/min. The testing machine contains a lower member that is fixed and an upper member that is removable. The specimen was fixed to the lower member and a metal bar with knife edge end was attached to the upper member in a direction at a right angle to the specimen. When the force was applied on the upper member, it was slide downward at the interface between the heat-cured and self-cured acrylic resin till separation of the self-cured portion. Shear bond strength was calculated as the maximum load during the shear test divided by the defined bonding area.

Where P is the applied load at fracture, and A is the cross-sectional area.

Transverse strength test

Fifty specimens of heat-polymerized acrylic resin were prepared using compression molding technique according to the manufacturer's instructions. The specimens were in the shape of flat-strips with dimensions of 65 mm (length) × 10 mm (width) × 2.5 mm (thickness) according to ADA specification no. 12 for denture base polymers [18]. After processing, the specimens were trimmed with an acrylic stone, polished and stored in water at 37°C for 7 days.

Preparation of fractured specimens

Bottom and sides of the specimens to be repaired were placed into dental stone (Elite dental stone; Zhermack, Badia Polesine, Italy) which is poured in a box to form repair index as shown in [Figure 4]. The specimens and index were numbered on the corresponding ends to allow realignment in the original position. A centering mark was made and two lines were drawn perpendicular to the long axis of the repair sample 1.5 mm on each side of the centering mark. The specimens were cut at these lines with a carbide bur, so 3 mm of the total length of the specimen were removed. Joint surface contours were made rounded with a carbide bur [3]. The fractured specimens were cleaned with distilled water and dried with air. The prepared 50 specimens were divided into five groups, 10 specimens each with the same grouping described for shear bond strength specimens.

After plasma treatment, the specimens were correctly oriented in their repair indices in such a way that a 3 mm gap exists between the two sections of the specimen. The edges of the specimens to be repaired were wetted by MMA monomer. An autopolymerized acrylic resin was mixed according to the manufacturer's instructions. The mix was flowed into the space, allowed to overfill it to compensate for polymerization shrinkage, pressed by glass slide to prevent porosity and finished. Specimens were stored in distilled water at 37°C for 24 h before testing.

Transverse strength testing

Transverse strength of the specimens was measured by three-point bending test using universal testing machine and data were recorded using computer software. The test rig consisted of a loading wedge and a pair of supporting wedges placed 50 mm apart (which represents the average inter-molar distance of a denture) with 5 kN load cell and a cross-head speed of 5 mm/min. This loading wedge has engaged the center of the upper surface of the specimen until the specimen has broken. The peak load at the moment of fracture was recorded. The transverse strength (S) of each specimen was determined using the formula:

Where P is the fracture load, L is the distance between two supports (50.0 mm), b is the specimen width, and d is the specimen thickness.

Statistical analysis

Mean values (MPa) and SD were calculated for each group and the results were statistically analyzed using the Graphpad Prism-4 statistics software for Windows (GraphPad Software, San Diego, California, USA). One-way analysis of variance (ANOVA) was performed to study the effect of plasma treatment on shear bond strength and transverse strength of repaired heat-cured acrylic resin. Tukey's multiple comparison tests were performed with 5% level of significance.

Results

Shear bond strength

The mean values (MPa) and SD for control and test groups are listed in [Table 1] and shown in [Figure 5]. One way ANOVA has revealed a significant difference between the mean values of shear bond strength (MPa) of repaired acrylic denture base resin (P < 0.05%).

The mean values (MPa) and SD for control and test groups are listed in [Table 2] and shown in [Figure 6]. One way ANOVA has revealed a significant difference between the mean values of transverse strength (MPa) of repaired acrylic denture base resin (P < 0.05%).

The objective of denture repair is to restore the denture's strength or reinforce it to avoid recurrent fractures. The repair procedure has to be rapid, easy to perform, inexpensive, does not change the original color, and preserves the dimensions of the denture. Several materials have been used to repair fractured acrylic resin, including heat-cured PMMA, autopolymerized acrylic resin, microwave-cured PMMA and visible light-polymerized resin. Autopolymerized resin is the repair material most commonly employed, which generally allows for a simple and quick repair [19]. Unfortunately, dentures repaired with autopolymerizing acrylic resin alone often experience a refracture at the repaired site [2].

The success of denture repair relies on the phenomenon of adhesion [20]. Strong bonding of the surfaces improves the strength of the repaired unit and reduces stress concentration [21]. Koodaryan and Hafezeqoran [22] have concluded that the technology of atmospheric plasma treatment can replace the traditional chemical and mechanical surface modifications in improving surface property and wettability of polymers. Ablation removes the contamination without leaving any organic residue. Wettability of the resin surface could be raised after plasma exposure, so that the plasma irradiation can make the surface washed, degreased, uneven and activated simultaneously. With plasma treatment, the surfaces of polymers can be improved in terms of hydrophilicity by forming oxygen-containing functional groups, such as C=O and –OH [15]. These effects result in acid–base interactions and covalent linkages. So, bonding enhancement is thus successfully achieved through plasma treatment [23]. In addition, Plasma irradiation does not cause environmental pollution, as it does not require chemicals. Therefore, it is a superior method to increase adhesive strength between heat-cured acrylic resin and self-curing acrylic resin [15].

Two kinds of plasma were used in this study; glow discharge plasma and DBD plasma. Glow discharge plasma system was used because of its low gas impurities, low consumption of power, high efficiency, and short time for treatment, flexibility of using different gases, treated materials, discharge powers, and gas pressure. DBD plasma system was used because of its low cost, high efficiency, short time of treatment, no vacuum system used, easy adjustability of discharge powers and pressures, easy operation and long working life-time. The pressure used was atmospheric pressure and ammeter reading (discharge current) was 2 mA. These parameters were used in the present study because they give the highest efficiency of the system and low running cost [24].

During specimen preparation, a small gap, 3 mm thickness, in the center of each sample was prepared. This helps to minimize the bulk of the repair and thus reduces the shrinkage. The small gap between the repaired joints also helps to make the three-point loading test calculation simple [21]. The repaired edges of the specimen were prepared as rounded bevel because it has the advantage of shifting the mode of fracture from a weak, adhesive interfacial fracture for the butt repair to a stronger cohesive fracture of the repair material in the rounded repair. The geometry of rounded repairs increases the interfacial bond area and shifts the interfacial stress pattern more toward a shear stress and away from the more damaging tensile stress exerted on the butt repair during flexure [3].

Measurement of the transverse strength is used in evaluation of denture plastics, as this strength closely resembles the type of loading applied to the denture in the mouth during mastication [25]. The fracture of repaired dentures often occurs at the junction of the previous and new material rather than within these materials [2]. The debonding of a repair material from a base resin could be evaluated by measuring the shear bond strength [17].

The results of the present study have showed that both types of plasma have increased the shear bond strength and transverse strength of repaired acrylic denture base resin. So, the first and second hypotheses are rejected. The highest mean value of shear bond strength was obtained with repaired acrylic denture base resin after treatment with glow discharge plasma at 720 J, followed by DBD plasma treatment at 360 J, Glow discharge plasma treatment at 360 J and dielectric barrier discharge plasma treatment at 720 J. Control group has recorded the lowest mean value of shear bond strength. Increase of the shear bond strength is in agreement with the study of Nishigawa et al. [15].

Bonding enhancement can be regarded as resulting from the overlap of following effects of plasma treatment: (a) surface cleaning, that is, removal of organic contamination from the surfaces; (b) ablation, or etching, of material from the surface, which can remove a weak boundary layer and increase the surface area; (c) cohesive strengthening of the polymeric surface by the formation of a thin cross-linked layer that mechanically stabilizes the surface; and (d) creation of chemical groups on the stabilized surface that result in acid–base interactions and in covalent linkages believed to yield the strongest bonds [23].

The results of shear bond strength have showed that the treatment with glow discharge is more efficient than the DBD because in glow discharge, the energy of the ions incident on the surface of the samples is higher than that of the ions in DBD which makes the ions in glow discharge are more able to etch the surface of the samples and hence increases the shear bond strength [26]. In addition, the etching process by ion bombardment is more efficient than that by electron bombardment because of the heavier mass of the ions. Also the electrons bombardment have heating effects on a thin layer of the surface of the treated samples, this heating effect eliminates the etching effect. In glow discharge, the samples are exposed mainly to ion bombardment. So, as the discharge energy increases from 360 to 720 J the etching process increase and hence shear bond strength increases. On the other hand, in DBD, the samples are exposed to ions bombardment and electrons bombardment. At low energy discharge, 360 J, the etching effect by ions bombardment dominates the heating effect by electrons bombardment. So, the shear bond strength increases compared with the control samples. At high energy discharge, 720 J, the heating effect by electrons bombardment dominates the etching effect by ions bombardment. So, the shear bond strength decreases compared with the samples treated at 360 J in DBD [27].

Increase of transverse strength of plasma treated specimens in comparison with control group could be attributed to the chemical and physical modifications produced on the surface when plasma is brought into contact with polymers [28]. This result is in agreement with the study of Yun et al. [29]. The study has reported that the degree of cross-linking on the polymer surface was enhanced after plasma treatment and, in addition to increased crystallinity, the cross-linked structure induced by plasma treatment restricted chain mobility. This effect could strengthen the polymer. However, studies of Awad and Jassim [30] and Pan et al.[12] have revealed a decrease in the transverse strength after plasma treatment. This could be attributed to the different type of plasma they have used and both studies have been performed on intact not repaired specimens.

Ion bombardment can break C–C or C–H bonds, and the free radicals resulting under these conditions can only react with other surface radicals or with other chains in chain-transfer reactions; therefore, they tend to be very stable [31]. If the polymer chain is flexible, or if the radical can migrate along it, this can give rise to recombination, unsaturation, branching, or cross-linking. The latter may improve the heat resistance and bond strength of the surface by forming a very cohesive skin [23]. While etching created by glow discharge plasma treatment has increased the shear bond strength, it has recorded lower flexural strength than DBD plasma because etching could create areas of stress concentration which may weaken the treated surface.